Overlay control for laser peening

ABSTRACT

A method of controlling the application of laser peening overlays on the surface of a workpiece to reduce the variability of shock waves generated therein, comprises applying an energy-absorbing overlay to a portion of the surface of a workpiece, measuring the thickness of the energy-absorbing overlay in at least one location on the energy-absorbing overlay, applying a transparent overlay material over the energy-absorbing overlay, measuring the thickness of the transparent overlay in at least one location on the transparent overlay, determining if the measured values for each overlay is within a specified range, and directing a pulse of coherent energy to the workpiece to create a shock wave therein when the measured values are within the specified range.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to the use of coherent energypulses, as from high-peak-power pulsed lasers used for the laser peeningof solid materials and, more particularly, to methods and apparatus tocontrol the application of the overlays applied for laser peening (alsocalled laser shock peening, laser shock processing, and shockprocessing). The invention is especially useful for reducing thevariability of the thickness of the overlays applied to the surface ofthe workpiece being laser shock peened.

[0003] 2. Description of the Related Art

[0004] Old methods for shock processing of solid materials typicallyinvolve the use of high explosive materials or high-pressure gases,which are used to accelerate a plate, that strikes the solid material toproduce shock waves therein. Such methods have several disadvantages.For example: (a) it is difficult and costly to shock process non-planarsurfaces and complicated geometries, (b) storage and handling of thehigh explosive materials and high pressure gases pose a hazard, (c) theprocesses are difficult to automate and thus fail to meet someindustrial needs and (d) high explosive materials and high pressuregases cannot be used in extreme environments such as high temperaturesand high vacuum.

[0005] Shot peening is another widely known and accepted process forimproving the fatigue, hardness, and corrosion resistance properties ofmaterials by impact treatment of their surfaces. In shot peening, manysmall shot or beads are thrown at high speed against the surface of amaterial. The shot or beads sometimes escape from the treatmentequipment and scatter in the surrounding area. Since the shot or beadsmight get into surrounding machinery and cause damage, shot peeningusually cannot be used in a manufacturing line. Ordinarily such shotpeening cannot be used on machined surfaces without a high likelihood ofdamaging them. In addition, shot peening has problems maintainingconsistency of treatment caused by inherent wear of the shot by the shotpeening equipment.

[0006] Laser peening equipment, however, can be incorporated intomanufacturing lines without danger to surrounding equipment. Laserpeening with coherent radiation has several advantages over what hasbeen done before. For example, the source of the radiation is highlycontrollable and reproducible. The radiation is easily focused onpreselected surface areas and the operating mode is easily changed. Thisallows flexibility in the desired shock (peening) pressure and carefulcontrol over the workpiece area to be laser peened. Workpieces immersedin hostile environments, such as high temperature and high vacuum can beshock processed. Additionally, it is easy to laser peen the workpiecerepetitively. This is desirable where it is possible to enhance materialproperties in a stepwise fashion. Laser peening the workpiece severaltimes at low pressures can avoid gross deformation, cracking, andspallation of the workpiece while nonplanar workpieces can be laserpeened without the need of elaborate and costly shock focusing schemes.

[0007] Laser peening utilizes two overlays: a transparent overlay(usually water and hereinafter referred to as overlay water) and anenergy absorbing opaque overlay; an oil based paint; an acrylic basedblack paint; or an energy-absorbing tape (and hereinafter referred to aspaint). During processing, the paint is locally applied to the surfaceof the workpiece to be processed, followed by the application of theoverlay water. A laser beam is directed to pass through the overlaywater and is absorbed by the paint, causing a rapid vaporization of athin layer of the paint and the generation of a high-amplitude shockwave in the workpiece. The shock wave cold works the surface of theworkpiece and creates compressive residual stresses, which provide anincrease in fatigue properties of the workpiece. A workpiece istypically processed by laser peening a matrix of overlapping spots thatcover the fatigue critical zone of the workpiece.

[0008] The method of rapidly applying an energy absorbing overlay orpaint, followed by a transparent overlay or overlay water, subsequentlylaser shock peening the workpiece, and then removing the overlay ispresented in U.S. Pat. No. 5,471,559. This method has been successfullyimplemented into laser peening equipment and has dramatically decreasedthe processing time and cost; however, the variability in thethicknesses and/or uniformity of the paint or overlay water canpotentially lead to differences in the resultant residual stresses inthe workpiece and thus change the fatigue life capabilities of thelaser-peened workpiece.

[0009] To provide a consistent laser shock peening process, theapplication of the paint and overlay water need to be monitored duringprocessing and subsequently controlled to ensure that the thicknessand/or uniformity of the paint and overlay water are the same for eachlaser shot. What is needed is a method to monitor and control theapplication of the paint and overlay water during processing to apply aconsistent thickness and/or uniformity for each laser shot.

SUMMARY OF THE INVENTION

[0010] The present invention provides a method of laser peening that canbe used in a production environment to significantly reduce thevariability of laser peening by monitoring and controlling the thicknessand/or uniformity of both the paint and the water overlays. The methodincludes the steps of painting the workpiece to be laser peened with alayer of paint applied to a small area about 2 to 3 times the diameterof the laser-peening spot, measuring the thickness and/or uniformity ofthe paint prior to the application of the overlay water, and recordingthese measured values. Subsequently, overlay water is applied, forming athin, flowing layer over the previously painted portion, and thethickness and/or uniformity of the overlay water is measured at aspecific time after its initial application, but prior to firing thelaser, and the measured values are recorded. The thickness of the paintor overlay water is the distance between the exposed surface of thepaint or overlay water and the surface upon which it is applied, and theuniformity is the difference in the thickness between two or morelocations on the overlay. The measured values for the paint and wateroverlay are then compared to the specified range. The specified rangemay be a predetermined range based upon engineering judgment, or it maybe determined by statistical methods. If the measured values are withinthe specified range, then the laser beam is directed through the wateroverlay and onto the painted area. If the measured values for the paintand water overlay thickness and/or uniformity are not within thespecified range for the process, then the laser is not allowed to fireand the high-speed water jet is activated to wash the paint spot off ofthe workpiece. The automated process of applying paint and overlay watermay be repeated without changes. Alternately, adjustments in theoverlay-application parameters or applicator head positions may be madeto change the thickness and/or uniformity of the overlays to bring theminto the specified range. Adjustments may be made by the controllingcomputer that sends signals to electronically controlled valves to alterthe flow rate, pressure, and application duration of the paint andoverlay water applicators, to bring the thickness and/or uniformityvalues for the two overlays into the specified range for the process.After the adjustments are made to the overlay-application parameters orapplicator head positions, the process sequence is restarted.

[0011] In the present application, the term “specified range” is thebounds that the thicknesses of the overlays need to be within forcontinued laser peening operations. “Specified” in the broadest sensemay mean limit values created with or without knowledge or historicaldata on successful laser peening. The term “preselected range” is afixed range that has been calculated before laser peening commences. Theterm “statistically determined range of values” is the limit rangescalculated “on the fly” and between each laser peening operation basedupon the same ongoing running total or historical data collected andanalyzed. For example, the ranges could be calculated as ± one standarddeviation from the mean of the last 100 thickness samples. This exampleis not limiting, other statistical methods and functions may be utilizedto create operational bounds or limits to effect overlay thicknesscontrol.

[0012] The invention comprises, in one form thereof, a method ofcontrolling the thickness of the paint and overlay water. The methodinvolves applying the paint and then the overlay water to the workpieceto be processed based upon thickness. The paint is first applied to aportion of the workpiece. During the application of the paint overlay,the thickness is measured in real time. Upon reaching the specifiedrange for the thickness, a signal is sent from the control computer toterminate the application of the paint and initiate the application ofthe overlay water. The thickness and/or uniformity of the overlay waterare monitored, and when the measured values are within the specifiedrange, the laser is fired and the laser beam is directed through theoverlay water and onto the painted area.

[0013] The invention comprises, in one form thereof, a method ofapplying a continuous flow of overlay water to the surface of theworkpiece during the laser shock peening process. The overlay water iscontinuously directed over the surface of the workpiece to be processed.The application of the overlay water is accomplished with an applicatorin direct contact with the workpiece or a high-speed water jet. A jet ofair is directed toward the overlay water near the area where the paintis to be applied to divert the overlay water to other areas on theworkpiece. The air jet is maintained during the application of thepaint. The thickness of the paint is measured and when it is within thespecified range, the air jet is turned off to allow the overlay water toreform and flow over the paint. The thickness of the overlay water ismeasured and when it is within the specified range, the laser is firedand the laser beam is directed through the overlay water and onto thepainted area.

[0014] The invention comprises, in one form thereof, a method ofcontrolling the thickness of the energy absorbing overlay with a tampingdevice such as an air jet that is directed to the energy absorbingoverlay. The position and operation of the air jet may be determined foreach workpiece through trial and error experimentation; or preferably,the position and operation of the air jet may be automated. If the airjet is automated, the position and operational parameters of the air jetmay be controlled in real-time or may be established through acalibration sequence prior to processing workpieces. In an automatedprocess, the thickness of the energy absorbing overlay is monitoredduring or after its application to the surface of the workpiece. If thethickness is not within the specified range, the control computertriggers the operation of at least one tamping device such as a jet ofair (or other suitable gas) directed toward the paint to conform it tothe proper thickness. The air jet or jets may be turned off prior to theapplication of the water overlay. The tamping device may also be amechanical tamping block that physically contacts the overlay.

[0015] The invention comprises, in a further form of the previousdescription, the use of air jets or nozzles to control the thickness ofthe water overlay.

[0016] The invention comprises, in yet another form thereof, anapparatus for measuring the thickness of the paint or overlay water thatare used for improving properties of a workpiece by providing shockwaves therein. The apparatus includes a device to measure the thicknessof the paint or overlay water applied to the surface of the workpiecewith overlay applicators. Means to control the flow of the overlaymaterials are provided. A tamping device operatively associated with themeasurement device and control unit is used to change the thickness ofthe overlay on the surface of the workpiece. A laser peening system isoperatively associated with the control device to provide a laser beam,which is directed through the overlay water to create a shock wave inthe workpiece. The measurement device, material applicators, materialapplicator flow control means, tamping device and laser peening system,are connected to a control computer that controls the operation andtiming of each of the applicators, measurement devices, control valves,tamping devices, and the laser.

[0017] The control computer collects the measured thickness values andcompares these values to the specified values for the laser peeningoperation on the workpiece being processed. The control computerdetermines if the values for an overlay are within the specifiedtolerance range. If they are within the specified range, the laseroperatively associated with the process is fired to generate a shockwave on the surface of the workpiece. If these measured values are notwithin the specified thickness range, the computer prevents the laserbeam from being fired and sends a signal to remove the paint and overlaywater. The control computer makes adjustments to the flow of the overlayapplicator that is out of the specified range, reapplies the paint andoverlay water, again measuring the thickness of the paint and overlaywater at specific points during or after application. The controlcomputer may also activate a tamping device such as an air jet to changethe thickness or smooth the paint or overlay water.

[0018] An advantage of the present invention is that it provides amethod to ensure a consistent thickness of an overlay, which providesmore consistent and repeatable laser peening results within theworkpiece.

[0019] Another advantage of the present invention is that the controlcomputer can control the timing of the application of the paint andoverlay water by measuring the thickness during application of theseoverlays.

[0020] Yet another advantage of the present invention is that thecontrol computer can make adjustments to the control valves to modifythe thickness of the paint and overlay water.

[0021] A further advantage of the present invention is that the controlcomputer can modify the thickness of the overlays applied to the surfaceof the workpiece by use of an tamping device.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] The above-mentioned and other features and advantages of thisinvention, and the manner of attaining them, will become more apparentand the invention will be better understood by reference to thefollowing description of an embodiment of the invention taken inconjunction with the accompanying drawings, wherein:

[0023]FIG. 1 is a diagrammatic view of one embodiment of the presentinvention;

[0024]FIG. 2 is a diagrammatic view of the measurement device relativeto the position of the overlay to be measured;

[0025]FIG. 3 is a flow chart of the method of the present invention formeasuring the thickness and uniformity of the paint and overlay water;

[0026]FIG. 4 is a flow chart of another method of the present inventionfor measuring the thickness and uniformity of the paint and overlaywater;

[0027]FIG. 5 is a flow chart of yet another method of the presentinvention for measuring the thickness and uniformity of the paint andoverlay water;

[0028]FIG. 6 is a series of views (A-D) of depicting the application ofthe energy absorbing coating while continuously applying water overlaywith a water jet;

[0029]FIG. 7 is a diagrammatic view of applying a continuously applyingwater overlay with a brush applicator in contact with the workpiece; and

[0030]FIG. 8 is a diagrammatic view of one embodiment of a tampingdevice used to modify the thickness of an overlay.

[0031] Corresponding reference characters indicate corresponding partsthroughout the several views. The exemplification set out hereinillustrates one preferred embodiment of the invention, in one form, andsuch exemplification is not to be construed as limiting the scope of theinvention in any manner.

DETAILED DESCRIPTION OF THE INVENTION

[0032] The improvements in fatigue life produced by laser peening arethe results of residual compressive stresses developed in the irradiatedsurface retarding fatigue crack initiation and/or slowing the crackpropagation rate. A crack front is the leading edge of a crack as itpropagates through a solid material. Changes in the shape of a crackfront and slowing of the crack growth rate when the crack frontencounters the laser shock zone in a laser peening condition have beenshown. Laser peening is an effective method of increasing fatigue lifein metals by treating fatigue critical regions. As to what effect thetensile residual stresses surrounding the laser shocked region wouldhave on crack initiation, a previous study is described in “Shock Wavesand High Strained Rate Phenomena in Metals” by A. H. Clauer, J. H.Holbrook and B. P. Fairand, ed. by M. S. Meyers and L. E. Murr, PlenumPress, New York (1981), pp. 675-702. Described in the above referenceare the effects of laser peening on fatigue of welded aluminum specimensthat had used a series of overlapping spots to cover the weld andheat-affected zones. Significant increases in fatigue life were observedfor these specimens, indicating that overlapping the spots did notsignificantly reduce the effects of laser shocking. This is supported byresults on a low carbon steel that showed no change in the compressiveresidual stress profile across the edge of a spot in a spot-overlapregion.

[0033] For a more thorough background in the prior history of laserpeening and that of high power processing of engineered materials,reference can be made to U.S. Pat. No. 5,131,957, such patent explicitlyhereby incorporated by reference. This patent also shows a type of laserand laser circuit adaptable for use with the present invention. Anothertype of laser adaptable for use with the invention is that of a Nd:GlassLaser manufactured by LSP Technologies of Dublin, Ohio.

[0034] Overlays are applied to the surface of the workpiece being laserpeened. These overlay materials may be of two types, one transparent tolaser radiation and the other opaque to laser radiation. They may beused either alone or in combination with each other, but it is preferredthat they be used in combination with the opaque overlay adjacent theworkpiece, and the outer, transparent overlay being adjacent the opaqueoverlay.

[0035] The transparent overlay material should be substantiallytransparent to the radiation. Useful transparent overlay materialsinclude water, water-based solutions, other noncorrosive liquids, glass,quartz, sodium silicate, fused silica, potassium chloride, sodiumchloride, polyethylene, fluoroplastics, nitrocellulose, and mixturesthereof. Fluoroplastics, as they are known by ASTM nomenclature, areparallinic hydrocarbon polymers in which all or part of each hydrogenatom has been replaced with a fluorine atom. Another halogen, chlorine,can also be part of the structure of a fluoroplastic. By order ofdecreasing fluorine substitution and increasing processability, thesematerials include polytetrafluoroethylene (PTFE); fluorinatedethylenepropylene (FEP): the chlorotrifluorethylenes (CTFE); andpolyvinylidine fluoride (PVF.sub.2). Also available is a variety ofcopolymers of both halogenated and fluorinated hydrocarbons, includingfluorinated elastomers. Additionally, the transparent overlay could be agel or a strip of tape comprised of one or more of the above materials.In the preferred embodiment of the present invention, water is used asthe transparent overlay to confine the plasma.

[0036] Where used, the opaque overlay material should be substantiallyopaque to the radiation. Useful opaque overlay materials include blackpaint, pentacrythritol tetranitrate (PETN); bismuth, lead, cadmium, tin,zinc, aluminum, graphite; and mixtures of charcoal or carbon black withvarious transparent materials such as mixtures of nitrocellulose andpotassium perchlorate or potassium nitrate. Optionally, a layer ofanother solid overlay material may be attached to the layer ofsubstantially opaque material. The outer, solid layer may be eithertransparent or opaque. The term “transparent” in this application isdefined as meaning pervious to the laser beam utilized, notautomatically or necessarily pervious to visible light. A typicaloverlay is between 10 micrometers and 20,000 micrometers (_m) thick. Inthe preferred embodiment of the invention, water based black paint isused to give superior results both in terms of energy absorption andremovability after laser peening.

[0037] Referring now to the drawings and particularly to FIG. 1, thereis shown a preferred embodiment 10 of the present invention including alaser-peening chamber 12 in which the laser peening takes place. Thelaser-peening chamber 12 includes an opening 14 for a laser beam 16created by laser 18, a source of coherent energy. Laser 18, by way ofexample, may be a commercially available high power pulse laser systemcapable of delivering more than approximately 10 Joules in 5 to 100nanoseconds. The laser beam energy, pulse length and spot size on theworkpiece may be adjusted as known in the art. Shown in FIG. 1, aworkpiece 20 is held in position within laser-peening chamber 12 bymeans of a positioning mechanism 22. Positioning mechanism 22 may be ofthe type of a robotically controlled arm or other apparatus to preciselyposition workpiece 20 relative to the operational elements of lasershock system 10.

[0038] Laser peening system 10 may include a material applicator 24 forapplying an energy absorbing material onto workpiece 20 to create acoated portion. Material applicator 24 may be that of a solenoidoperated painting station or other construction such as a jet spray oraerosol unit to provide a small, coated area onto workpiece 20. Thematerial utilized by material applicator 24 is an energy absorbingmaterial, preferably that of a black, water-based paint such as 1000 FAQUATEMP(™) from Zynolite Product Company of Carson, Calif. Anotheropaque overlay that may be utilized is that of ANTI-BOND, a watersoluble gum solution, including graphite and glycerol from MetcoCompany, a Division of Perkin-Elmer of Westbury, N.Y. Alternatively,other types of opaque overlays may be used such as those discussedabove.

[0039] Laser peening system 10 further includes a transparent overlayapplicator 26 that applies a fluid or liquid transparent overlay toworkpiece 10 over the portion coated by opaque overlay applicator 24.The transparent overlay material should be substantially transparent tothe radiation as discussed above, water being the preferred overlaymaterial.

[0040] Laser peening system 10 also includes a measurement device 15 tomeasure the thickness of the paint and overlay water. The measurementdevice is preferably a non-contact device that is capable of measuringthe paint and overlay water thickness from a distance and will notinterfere with the operation of the applicator heads 24 and 26.Referring to FIG. 2, a diagrammatic view of one arrangement of therelative position of the measurement device 15 to the surface of theoverlay surface to be measured 40 on workpiece 20 that does notinterfere with the propagation of laser beam 16 or the operation of theoverlay applicator head 24, 26 is shown. This measurement device may bethat of an image processing camera or laser thickness measurementsystem, for example, a CCD laser displacement sensor made by Keyencemodel LK-503 of Woodcliff Lake, N.J.

[0041] An image processing system using a subtraction algorithm (imagebefore and after the overlay is applied) may be used to provide aplurality of thickness measurements over the entire area to beirradiated with the laser pulse. Alternatively, other single-pointmeasurement devices may be scanned over the area to obtain a pluralityof thickness measurements. The plurality of thickness measurements maythen be used to obtain a quantitative measure of the uniformity of theoverlay. The use of the term uniformity means a plurality of thicknessmeasurements mathematically combined to determine a measure of thevariation of thickness across the overlay. In one simple form theuniformity may be expressed as the standard deviation of the thicknessmeasurements. The overlay is considered to be acceptable if both thethickness and the Standard deviation (uniformity) are within apredetermined range.

[0042] It is also important to note that, in this preferred embodimentshown in FIG. 1, measurement device 15 is capable of measuring thethickness of both paint and water. However, it may be necessary to usetwo measurement devices (15, 15′), one for measuring the opaque-overlaythickness and one for measuring transparent-overlay thickness.

[0043] Laser peening system 10 includes tamping device 25 to change thethickness and/or uniformity of the paint or overlay water. The tampingdevice may direct a controlled pulse of gas to the surface of theapplied overlays to alter the thickness and uniformity of the appliedoverlays. Alternatively, the gas may be directed to the workpiecesurface near the overlay to affect the thickness and uniformity of theoverlay. The gas may be compressed air or other compressed gassesdirected through the nozzle.

[0044] As shown in FIG. 1, both opaque overlay applicator 24 andtransparent overlay applicator 26 are shown directly located withinlaser-peening chamber 12 along with the measurement device 15 andtamping device 25. In a production operation environment, only thenecessary operative portions need be located through and withinlaser-peening chamber 12 such as the portion through which the overlaymaterials actually flow onto the workpiece. The supply tanks, pumps andother auxiliary equipment for applying the transparent overlay materialsand energy absorbing overlay materials may be located outside oflaser-peening chamber 12. The location of the measurement device may belocated either near the workpiece 20 being processed or outside chamber12 with a clear path to the area being processed on workpiece 20.Tamping device 25 needs to be located in close proximity to theworkpiece 20 preferably near applicator heads 24 and 26.

[0045] A control unit, such as controller 28 is operatively associatedwith each of the opaque overlay material applicator 24, transparentoverlay material applicator 26, measurement device 15, tamping device25, laser 18 and positioning mechanism 22. Controller 28 controls theoperation and timing of each of the applicators 24, 26, tamping device25, laser 18 and selective operation of positioning mechanism 22 toensure proper sequence and timing of laser peening system 10. Inaddition, controller 28 acquires the thickness and uniformitymeasurements from measurement device 15. Shown in FIG. 1, controller 28is connected to laser 18, positioning mechanism 22, opaque overlaymaterial applicator 24, transparent overlay material applicator 26,measurement device 15 and tamping device 25 via control lines 30, 32, 3436, 17 and 27, respectively. Controller 28 is also connected to controlvalves 29 and 35 via lines 31 and 33 respectively, and to controlmaterial input lines 19 and 21 for applicators 24 and 26, respectively.Controller 28, in one embodiment, may be a programmed personal computeror microprocessor.

[0046] In operation, controller 28 controls operation of laser peeningsystem 10 once initiated. As shown in FIG. 3, the method of theinvention is that first, workpiece 20 is located (38) particularlywithin laser-peening chamber 12 by positioning mechanism 22. Controller28 activates material applicator 24 to apply an energy absorbing overlay(40) such as a water-based black paint onto a particular location ofworkpiece 20 to be laser peened. Next the controller acquires athickness and/or uniformity measurement of the paint (42) frommeasurement device 15 and stores the value in controller 28. The nextstep of the process is that controller 28 causes transparent overlaymaterial applicator 26 to apply transparent overlay (44) to thepreviously coated portion of workpiece 20. The controller acquires athickness measurement of the transparent overlay (46) and stores thisvalue in controller 28. At this point, controller 28 compares themeasured thickness and uniformity values of the paint and overlay waterto the specified values (48) for each. These specified values may be apredetermined range of values based upon processing conditions or aspecified range, which may be based upon a statistically determinedrange. If the measured values for the two overlays are within specifiedrange (52), laser 18 is immediately fired (52) by controller 28 toinitiate a laser beam 16 to impact the coated portion. If the measuredvalues are not within the specified range (56) the controller 28initiates a wash sequence and removes the overlays (56). The controllerthen makes adjustments to the applicator head valves 29 and 35 toachieve the desired thickness and uniformity values (58).

[0047] In operation, controller 28 controls operation of laser peeningsystem 10 once initiated. As shown in FIG. 4, the method of theinvention is that first, workpiece 20 is positioned at the properlocation within laser-peening chamber 12, by positioning mechanism22(60). Controller 28 activates overlay applicator 24 to apply alaser-energy absorbing overlay (62) such as paint onto a particularlocation of workpiece 20 to be laser peened. The controller 28 acquiresa thickness and/or uniformity measurement (64) of the paint frommeasurement device 15 and terminates the applications of the overlaywhen the thickness is within the specified range (66). The uniformity ofthe energy absorbing overlay is compared to a specified range (67). Whenthe thickness and uniformity are within the specified range (68), theapplication of the energy-absorbing overlay is terminated and thecontroller 28 applies the overlay water (70). The controller acquires athickness and uniformity measurement (72) of the overlay water andcompares them to the specified range (74). If the uniformity of thelaser-energy absorbing overlay is not within the specified range (76),the application of the laser-energy absorbing overlay is terminated (78)and it is removed (80) and the application of the energy absorbingoverlay is restarted (82). If the thickness and uniformity are withinthe specified range (68), the controller 28 applies the overlay water(70) and then measures the thickness and uniformity of this overlay (72)with measurement device 15. The controller 28 compares these values tothe specified range (74). If the thickness and uniformity of the overlaywater is not within the specified range (90), the controller 28terminates the application of the overlay water (92) and initiates theoverlay removal (94). If the thickness and uniformity of the overlaywater are within the specified range (84), laser 18 is immediately fired(86) by controller 28 and laser beam 16 is directed to the workpiece togenerate a shock wave and then the process is restarted (88).

[0048] In operation, controller 28 controls operation of system 10 onceinitiated. As shown in FIG. 5, the method of the invention is thatfirst, workpiece 20 is located (100) particularly within laser-peeningchamber 12 by positioning mechanism 22. FIG. 6 shows a sequence of themethod for one type of arrangement. Controller 28 activates overlaywater applicator 26 to apply a continuous flowing stream (42) of overlaywater (102) to the surface of workpiece 20. The stream 40 impinges onthe surface of work piece 20 and produces a layer of water 42 on thesurface of the work piece in the shape of a parabola. The air jet 25directed toward the area on the workpiece to be processed is activated(104) in order to divert the stream of overlay water (44) flowing overthe area to be processed on the workpiece 20. The controller 28activates a material applicator 24 to apply paint 46 (106) to thesurface of the workpiece 20. The controller 28 continuously measures thethickness (108) of the paint 48 on workpiece 20 with measuring device 15during the application and upon reaching the designated thickness, thecontroller 28 terminates the application (110) of the paint frommaterial applicator 24. Controller 28 compares the measured thicknessand uniformity values to the specified range (111) and if they arewithin the specified range, the controller 28 terminates the jet of air52 (112) coming from air jet nozzle 25 to allow the overlay water toflow over the newly applied paint. Controller 28 acquires thickness anduniformity measurements (114) from measurement device 15 of the overlaywater on the area to be processed on workpiece 20. The thickness anduniformity values are compared to a specified range (116) and if themeasured values are within the specified range (118), laser 18 isimmediately fired (120) by controller 28 and laser beam 16 is directedto the workpiece. This sequence decreases total cycle time between laserpeening operation, thereby reducing processing costs.

[0049] An alternate method of applying the water overlay to the surfaceof the part is to use a brush applicator 52 as shown in FIG. 6 that hasbristles 54 in contact with work piece 20 to produce a layer of water 42on the surface of work piece 20. The water overlay 42 is diverted withthe air jet device 25 to allow the applicator 24 to apply theenergy-absorbing coating.

[0050] Although a brush applicator 52 with bristles 54 is disclosed,other equipment and means may equivalently be utilized to produce acontrolled layer of water 42. Other equivalent structures would utilizethe surface tension of the water to control application and distributionof the water flow, such as sponges, wipers, members with multipleopenings or pores, or other structures to create a substantial flatsheet of water on the workpiece.

[0051] Additionally, the applicator may include a laminar, slow controlmeans to produce a substantially nonturbulent flow of water as appliedto the workpiece. One structure for creating a laminar flow would be aplurality of adjacent then walled tubes or conduit through which thewater would be conducted to workpiece. The thin walled tubes may beoriented and spaced so that their output ends conform to a surface ofthe workpiece.

[0052] Additional to the basic method of operation, it may beadvantageous to manipulate the thickness and uniformity of the paint andoverlay water applied to the surface of the workpiece 20 to beprocessed. When the measurements of the thickness and/or uniformity ofthe paint and/or overlay water do not fit within the target range, thecontroller 28 can activate the air jet nozzle 25 to deliver preciseduration and pressure of a jet of air to the paint and or overlay waterto tamp it to the desired thickness and uniformity on the workpiece 20.As shown in FIG. 7, a device to tamp the overlays is composed of anozzle 200 mounted on a computer controlled gimbal 204 to allow thenozzle to be directed around axis 224 and 228 as instructed bycontroller 28 through control line 206 to tamp the overlay in any areaas needed. The tamping device is connected to a hose 212 that suppliescompressed gas to the nozzle 200 when solenoid 208 is activated bycontroller 28 through control line 216. The solenoid 208 is connected toa compressed gas supply by hose 220.

[0053] The above-described process or portions of the process arerepeated to laser peen the desired surface area of workpiece 20.Depending upon the energy levels and the amount of laser peening desiredon workpiece 20, controller 28 may position or re-index workpiece 20into another position using positioning mechanism 22, so that laserpeening system 10 may apply overlays to a different portion of thesurface, which may overlap the previously laser-peened area.

[0054] While this invention has been described as having a preferreddesign, the present invention can be further modified within the spiritand scope of this disclosure. This application is therefore intended tocover any variations, uses, or adaptations of the invention using itsgeneral principles. Further, this application is intended to cover suchdepartures from the present disclosure as come within known or customarypractice in the art to which this invention pertains and which fallwithin the limits of the appended claims.

What is claimed is:
 1. A method of controlling the application of laser peening overlays on the surface of a workpiece to reduce the variability of shock waves generated therein, comprising: applying an energy-absorbing overlay to a portion of the surface of a workpiece; measuring the thickness of said energy-absorbing overlay in at least one location on said energy-absorbing overlay; applying a transparent overlay material over said energy-absorbing overlay; measuring the thickness of said transparent overlay in at least one location on said transparent overlay; determining if said measured values for each said overlay is within a specified range; and directing a pulse of coherent energy to said workpiece to create a shock wave therein when said measured values are within the specified range.
 2. The method of claim 1 in which said specified range is a preselected range of values.
 3. The method of claim 1 in which said specified range is a statistically determined range of values.
 4. The method of claim 1 in which said overlays are removed from said surface of the workpiece when any one of the said measured values are not within the specified range.
 5. The method of claim 1 further comprised of measuring the uniformity of the surface of said energy absorbent overlay and comparing the measured value of said uniformity to a specified range.
 6. The method of claim 1 further comprised of measuring the uniformity of the surface of the transparent overlay and comparing the measured value of said uniformity to a specified range.
 7. The method of claim 1 in which said energy absorbing overlay and said transparent overlay are removed from said surface of said workpiece after said coherent pulse of energy has been delivered to said surface.
 8. The method of claim 1 wherein said method is repeated to another portion of the surface of said workpiece.
 9. The method of claim 1 wherein said step of applying an energy-absorbing overlay comprises applying a water based paint.
 10. The method of claim 1 wherein said step of applying a transparent overlay comprises applying a layer of water.
 11. The method of claim 1 in which said step of applying a transparent overlay comprises applying a layer of distilled water to the surface of said workpiece.
 12. The method of claim 1 in which said step of applying a transparent overlay comprises applying a layer of de-ionized water.
 13. The method of claim 1 in which all of the steps are timed and controlled by an electronic microprocessor.
 14. The method of claim 1 in which the process is repeated about every 0.1 to 10.0 seconds.
 15. The method of claim 1 further comprised of storing said measured values in an electronic microprocessor and comparing said measured values to said specified range.
 16. The method of claim 1 further comprised of using an electronic microprocessor to control one of the pressure and flow of one of the energy-absorbing overlay and overlay water.
 17. A method of improving properties of a solid material by providing shock waves therein, comprising: applying an energy absorbing overlay to a portion of the surface of the solid material; measuring the thickness of said energy absorbing overlay during said application; terminating the application of said energy absorbing overlay upon reaching the specified range; applying the transparent overlay over said energy absorbing overlay; measuring the thickness of said transparent overlay; and when the thickness of said transparent overlay is in the specified range, directing a pulse of coherent energy to said coated portion of the solid material to create a shock wave.
 18. The method of claim 17 in which said specified range is a predetermined range.
 19. The method of claim 17 in which said specified range is a statistically determined range.
 20. The method of claim 17 further comprised of measuring the uniformity of said energy absorbing overlay and terminating its application upon reaching a specified range.
 21. The method of claim 20 in which the specified range is a predetermined range.
 22. The method of claim 20 in which the specified range is a statistically determined range.
 23. The method of claim 17 further comprised of measuring the uniformity of said transparent overlay and terminating its application upon reaching a specified range.
 24. The method of claim 23 in which the specified range is a predetermined range.
 25. The method of claim 23 in which the specified range is a statistically determined range.
 26. The method of claim 17 in which the process is repeated about every 0.1 to 10.0 seconds.
 27. A method of improving properties of a solid material by providing shock waves therein, comprising: applying a continuously flowing transparent overlay to a portion of the surface of the solid material; directing an air jet to divert said transparent overlay from said portion of said solid material; applying energy absorbing overlay to said portion of the solid material; terminating said air jet to allow said transparent overlay to again be applied to said portion and over said energy absorbing overlay; and directing a pulse of laser energy to said portion of the solid material to create a shock wave.
 28. The method of claim 27 wherein said continuous flow of transparent overlay is applied through a brush in contact with the workpiece.
 29. The method of claim 27 wherein said continuous flow of transparent overlay is applied with a high-speed water jet.
 30. The method of claim 27 further comprised of terminating the application of said energy absorbing overlay when said thickness is within the specified range.
 31. The method of claim 27 further comprised of directing a pulse of laser energy when said thickness of said transparent overlay is within the specified range.
 32. The method of claim 27 further comprised of measuring the uniformity of said energy absorbing overlay and terminating its application upon reaching a specified range.
 33. The method of claim 27 further comprised of measuring the uniformity of said transparent overlay and directing a pulse of laser energy when said uniformity of said transparent overlay is within the specified range.
 34. The method of claim 27 in which the process is repeated about every 0.1 to 10.0 seconds.
 35. A method of modifying the thickness of the energy absorbing overlay and transparent overlay applied to the surface of solid material, comprising: applying an energy absorbing overlay to a portion of the said solid material; measuring the thickness of said energy absorbing overlay; tamping said energy absorbing overlay to the desired thickness; applying the transparent overlay over said energy absorbing overlay; measuring the thickness of said transparent overlay; tamping said transparent overlay to the desired thickness; and directing a pulse of laser energy to said coated portion of the solid material to create a shock wave.
 36. The method of claim 35 wherein said tamping is accomplished by a jet of air.
 37. The method of claim 35 further comprised of comparing the thickness of said energy absorbing overlay to a specified range.
 38. The method of claim 35 further comprised of alternatively activating the air jet to tamp the thickness of said energy absorbing overlay and measuring the thickness of said overlay until the thickness of said overlay is within the specified range.
 39. The method of claim 35 further comprised of alternatively activating the air jet to tamp the thickness of said transparent overlay and measuring the thickness of the overlay until said thickness of said overlay is within the specified range.
 40. An apparatus for monitoring and controlling the thickness and uniformity of the energy absorbing overlay and the transparent overlay, in a laser peening system comprising: a material applicator for applying an energy absorbing overlay onto the a workpiece; a material applicator for applying a transparent overlay over said coated portion; a measurement device to measure the thickness of said energy absorbing overlay and said transparent overlay; a measurement device to measure the uniformity of said energy absorbing overlay and said transparent overlay; a tamping device to alter said thickness and said uniformity of said energy absorbing overlay and said transparent overlay; a laser operatively associated with said transparent overlay applicator to provide a laser beam through the transparent overlay to create a shock wave on the workpiece; and a control unit operatively associated with each of said applicator, each said measurement device, said damping device, and said laser, to control the operation and timing of each said applicator, said measurement device, said tamping device and said laser.
 41. The apparatus of claim 40 in which said control unit is a programmable personal computer.
 42. The apparatus of claim 40 further in which said measurement devices are optical measurement devices.
 43. The apparatus of claim 40 further comprising control valves for each said material applicator to control the pressure and flow of said energy absorbing overlay and said transparent overlay wherein each said control valve is operatively associated with said control unit.
 44. The apparatus of claim 40 wherein said tamping device is an air jet nozzle operatively associated with said control unit.
 45. The apparatus of claim 40 wherein a positioning mechanism is used to index said workpiece past each said material applicator and is operatively associated with said control unit.
 46. A method of controlling the application of a transparent overlay on the surface of a workpiece to reduce the variability of shock waves generated therein, comprising: applying a transparent overlay material over said surface of said workpiece; measuring the thickness of said transparent overlay in at least one location on said transparent overlay; determining if said measured value for said overlay is within the specified range; and directing a pulse of coherent energy to said workpiece to create a shock wave therein when said measured value is within the specified range.
 47. The method of claim 46 wherein said workpiece has an opaque overlay applied prior to laser peening.
 48. The method of claim 47 wherein said opaque overlay is paint.
 49. The method of claim 47 wherein said opaque overlay is tape.
 50. A method of controlling the application of an overlay on the surface of a workpiece to reduce the variability of shock waves generated therein, comprising: applying an overlay material over said surface of said workpiece; tamping the overlay with a tamping device to alter the thickness of said overlay; and directing a pulse of coherent energy to said workpiece to create a shock wave therein.
 51. The apparatus of claim 50 in which a control unit is operatively associated with said tamping device.
 52. The apparatus of claim 50 in which said control unit is a programmable personal computer.
 53. The apparatus of claim 50 wherein said tamping device is an air jet nozzle operatively associated with a control unit.
 54. The apparatus of claim 50 wherein said tamping device contacts the surface of the overlay operatively associated with a control unit.
 55. The method of claim 50 further comprised of measuring the thickness of the overlay with measuring devices operatively associated with a control unit.
 56. The method of claim 54 further comprised of measuring the thickness of the overlay at several locations operatively associated with a control unit.
 57. The method of claim 54 further comprised of comparing the thickness of said overlay to a specified thickness range.
 58. The apparatus of claim 54 in which said measurement devices are optical measurement devices.
 59. The apparatus of claim 50 wherein a positioning mechanism is used to index said workpiece past each said tamping device and is operatively associated with said control unit.
 60. The method of claim 50 wherein the thickness of said overlay is tamped to a desired thickness.
 61. The method of claim 50 further comprised of alternatively activating the air jet to tamp the thickness of said energy absorbing overlay and measuring the thickness of said overlay until the thickness of said overlay is within the specified range.
 62. The method of claim 50 wherein said overlay is a transparent overlay to said laser beam.
 63. The method of claim 61 wherein said transparent overlay is water.
 64. The method of claim 50 wherein said overlay is an opaque overlay to said laser beam.
 65. The method of claim 63 wherein the said opaque overlay is paint.
 66. The method of claim 63 wherein the said opaque overlay is tape.
 67. The method of claim 50 wherein said overlay is an water applied over the surface of the part previously coated with an opaque overlay.
 68. A method of controlling the application of laser peening overlays on the surface of a workpiece to reduce the variability of shock waves generated therein, comprising: applying an energy-absorbing overlay to a portion of the surface of a workpiece; measuring the thickness of said energy-absorbing overlay in at least one location on said energy-absorbing overlay; applying a transparent overlay material over said energy-absorbing overlay; determining if said measured value for said energy absorbing overlay is within a specified range; and directing a pulse of coherent energy to said workpiece to create a shock wave therein when said measured value is within the specified range.
 69. The method of claim 1 in which the application of the transparent overlay material is accomplished with a brush.
 70. The method of claim 17 in which the application of the transparent overlay material is accomplished with a brush.
 71. The apparatus of claim 40 in which said transparent overlay applicator is a brush.
 72. The apparatus of claim 40 in which said transparent overlay applicator includes a laminar flow means. 